Process for the catalytic dehydrogenation of a light alkane

ABSTRACT

The present invention relates to a new catalyst support material comprising a mixed oxide consisting essentially of a divalent metal and a trivalent metal in a substantially homogeneous phase, the mixed oxide being a calcination product of a hydrotalcite-like phase calcinated at a temperature of about 700-1200° C., wherein the divalent metal/trivalent metal molar ratio is greater than or equal to 2. The invention also relates to a process of preparing the support. The invention further provides a catalyst for dehydrogenation which includes a transition metal selected from the first row of transition metals of the periodic table and/or a Group VIII metal impregnated on the new catalyst support material. The invention also provides a process for dehydrogenation of light alkanes using the catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 09/096,988,filed Jun. 12, 1998, now U.S. Pat. No. 6,313,063, which is acontinuation in part of co-pending U.S. patent application Ser. No.08/569,185, filed on Dec. 14, 1995, now U.S. Pat. No. 5,817,596, whichis a 371 National Stage Application of international application No.PCT/NO94/00102, filed Jun. 1, 1994, and published as internationalpublication No. WO 94/29021 on Dec. 22, 1994.

FIELD OF THE INVENTION

The present invention relates to the preparation of highly stable, highsurface area catalyst carrier materials derived from hydrotalcite-typematerials by calcination at an elevated temperature.

BACKGROUND OF THE INVENTION

The dehydrogenation of paraffins to olefins is of considerablecommercial importance due to the need for olefins for the manufacture ofproducts such as high octane gasolines, synthetic elastomers,detergents, plastics, ion exchange resins and pharmaceutical products.For a dehydrogenation process to be commercially useful, it must utilizecatalysts exhibiting a high activity, a high rate of conversion, a highselectivity for the formation of olefins, and a high stability.

A large number of catalysts are previously known for the dehydrogenationof paraffins. These catalysts comprise a solid carrier material on aninorganic oxide base and various catalytic metals and promoter metalsdeposited on the carrier material or incorporated into the carriermaterial by other means. Carrier materials on an alumina base have beenwidely used in such dehydrogenation catalysts.

U.S. Pat. No. 4,788,371 discloses such catalyst and a process for thesteam dehydrogenation of dehydrogenatable hydrocarbons with oxidativereheating. A dehydrogenatable C₂₋₃₀ hydrocarbon, steam and anoxygen-containing gas are contacted in a reaction zone with a catalystcomprising a Group VIII noble metal, one or more components selectedfrom lithium, potassium, rubidium, cesium and francium, and a componentselected from boron, gallium, indium, germanium, tin and lead, depositedon an inorganic oxide carrier material. The preferred carrier materialis alumina having a surface area of 1-500 m²/g, preferably 5-120 m²/g.Alumina is employed as the catalyst carrier in all the working examplesof the patent. A preferred catalyst according to said U.S. patentcontains about 0.70 wt. % of platinum, about 0.50 wt. % of tin and about3.86 wt. % of cesium, and has a surface area of about 85 m²/g.

Mixtures of magnesium oxide MgO and alumina Al₂O₃ and mixed oxides of Mgand Al have also been utilized as catalysts, and as carrier materialsfor catalysts. International Patent Application No. PCT/JP89/00053discloses an alkoxylation catalyst comprising a magnesium oxide that hasbeen modified by adding thereto at least one trivalent metal ion,preferably selected from Al³⁺ and Ga³⁺. British Patent Application GB2,225,731 discloses a catalyst for hydrotreatment, e.g.hydrodemetallization or hydrodesulphurization, comprising in asubstantially homogenous phase magnesia and alumina wherein the molarratio of Mg to Al is preferably from 3:1 to 10:1, together with a GroupVI metal and/or at least one Group VIII metal.

Hydrotalcite is a layered mineral of formula: Mg₆Al₂(OH)₁₆CO₃4H₂O. Overthe years, a large number of hydrotalcite-like compounds, of generalformula: [M(II)_(1−x)M(III)_(x)(OH)₂]^(x+)(A^(n−) _(x/n)) mH₂O, whereA=anions, have been prepared. Cavani, F. et al., Cat. Today, vol.11,no.2, 173 (1991). These compounds are characterized by a sheet-likestructure, in which the anions are located in the interlayer between twobrucite-like sheets containing the metal ions. M^(II) ₁ M^(III) metalions having an ionic radius which is not too different from Mg²⁺ canform hydrotalcite-like compounds. Cavani, F. et al., supra.

Upon calcination at 400-700° C., a high surface area (typically 160-220m²/g) material with an XRD pattern typical for MgO is formed, withoutseparation of the two metal ions into separate oxide phases. Schaper,H., et al., Appl. Cat., vol. 54, 79 (1989). Upon calcination at evenhigher temperatures, the mixed oxide is gradually transformed into aspinel structure, i.e., M^(II)M^(III) ₂O₄₁ with a much lower surfacearea. McKenzie, A. L., et al., J. Catal., vol. 138, 347 (1992);Bellotto, M., et al., Phys. Chem., vol. 100, 8535 (1996). One major usefor these materials is as support materials for catalysts, (see, Cavani,F. et al., supra) for instance for the catalytic dehydrogenation oflower alkanes. Akporiaye, D. et al., Norwegian Patent No. 179131 (1993).It has been reported that certain materials formed by calcination of aMg—Al-containing hydrotalcite at 300-700° C. exhibit a high stabilitytowards sintering in a humid atmosphere. See, Schaper, H., et al., Appl.Cat., supra.; Schaper, H., European Patent No. 0 251 351 (1988).

SUMMARY OF THE INVENTION

The present invention provides a catalyst which has improved catalyticperformance compared to prior art catalysts with regard to catalystactivity, and at the same time exhibits an increased catalyst life timeby preventing irreversible deactivation like sintering of the support.

In one early embodiment of the invention described in co-pending U.S.patent application Ser. No. 08/569,185, it had been found that if amixed oxide of Mg and Al is used in combination with a Group VIII noblemetal and certain promoters of the kind disclosed in the above-mentionedU.S. Pat. No. 4,788,371, a catalyst can be obtained which exhibitsimproved activity and stability when used for dehydrogenatingdehydrogenatable hydrocarbons.

The carrier for that embodiment of the catalyst may be prepared byadding a solution of sodium hydroxide and sodium carbonate to a solutionof magnesium nitrate and aluminum nitrate according to the methoddescribed in Journal of Catalysis, vol. 94, pp.547-557, (1985),incorporated herein by reference. Instead of sodium hydroxide and sodiumcarbonate, potassium hydroxide and potassium carbonate can be used, seeApplied Catalysis, vol. 55, pp. 79-90 (1989), incorporated herein byreference. A hydrotalcite-like compound Mg₆Al₂ (OH)₁₆CO₃-4H₂O is formedby evaporation (drying) of the above-mentioned mixtures. Thehydrotalcite is then calcinated at a temperature 500-800° C. to giveMg(Al)O. The molar ratio of Mg to Al typically ranges from 1:1 to 10:1,and the surface area is typically ranging from 100 to 300 m² per gram,preferably from 140 to 210 m² per gram, and the particle size can be inthe range of 100 μm to 20 mm.

The calcination temperature for that embodiment of the catalyst waswithin the range of about 500 to about 800° C. A calcination temperaturethat had been shown to produce good results was about 700° C. In some ofthe examples set forth herein, this temperature was held for about 15hours.

It has now been found, however, that the stability of the catalystdescribed herein could be further improved.

Thus, the present invention provides for a catalyst support materialcomprising a mixed oxide consisting essentially of a divalent metal anda trivalent metal in a substantially homogeneous phase, which is acalcination product of a hydrotalcite-like phase calcinated at atemperature of about 700-1200° C., wherein the divalent metal/trivalentmetal molar ratio is equal to, or higher than 2.

Tests of the effect of the calcination temperature of hydrotalcite andhydrotalcite-like materials at different temperatures from 700° C. to1200° C. were therefore investigated.

By performing these investigations, it has been surprisingly found thatby raising the calcination temperature of the catalyst support precursorhydrotalcite to 700° C. to 1200° C., preferably to the range of 750 to950° C., an improvement of the catalyst stability could be achieved withan acceptable reduction in the surface of the catalyst carrier comparedto the gain in stability at use. In a further aspect, the presentinvention thus relates to a catalyst support material comprising a mixedoxide consisting essentially of Mg and Al in a substantially homogenousphase, which is a calcination product of a hydrotalcite phase,preferably calcinated at a temperature of 750 to 950° C., wherein theMg/Al molar ratio is equal 2 or higher than 2. A most preferred rangefor the calcination has been found to be at 770 to 850° C., and withinthat range, the preferred temperature is at about 800° C.

Preferably the Mg/Al molar ratio is in the range of about 2.5 to 6.0,and most preferably, the Mg/Al molar ratio is in the range about 3 toabout 5.

In another aspect of the present invention, a method for preparing saidcatalyst support material is provided wherein a solution comprising adivalent metal salt and trivalent metal salt is mixed with a basicaqueous solution, the reaction product recovered from said reactionmixture, said product being washed and dried, and the dried product iscalcinated at a temperature ranging from about 700-1200° C. Calcinationtemperatures in the range of 750-950° C. have been found particularlysuitable. More preferably the calcination takes place at a temperatureranging from about 770 to about 850° C. The best results have so farbeen achieved when the calcination was performed at about 800° C.

The calcination may be effected, for example, for about 1 to about 20hours, and preferably the calcination is effected for about 2-15 hours.

The basic aqueous solution used in this process is preferably acomposition of aqueous ammonium or alkali metal hydroxides andcarbonates.

The preferred divalent metal therein is Mg and the preferred trivalentmetal therein is Al.

The molar ratio of hydroxide to carbonate may, for example, be withinthe range of 1:1 to 3:1.

In another aspect, the present invention relates to a dehydrogenationcatalyst comprising a transition metal, preferably a transition metalselected from the first row of transition metals of the Periodic Tableand/or a Group VIII metal, impregnated on to the catalyst supportdescribed above.

Preferably the first row transition metal is Cr.

Preferably this catalyst comprises both a Group IVA metal and a GroupVIII metal impregnated onto the catalyst support material mentionedabove. Optionally a Group IA metal may be used together with the GroupVIII metal and the Group IVA metal.

Preferably the Group VIII is Pt, the Group IV metal is Sn and the GroupIA metal is Cs. Preferably the Group VIII metal catalyst is in the rangeof 0.05 to 5.0 percent by weight and the amount of the Group IVA metalis 0.05 to 7.0 percent by weight, optionally Group IA 0.05 to 5 percentby weight.

The present invention also relates to a process for the catalyticdehydrogenation of light alkanes wherein a stream of such light alkanesis passed through a layer of the catalytic active compositions describedabove in the presence or absence of steam.

Thus, according to one embodiment this process is performed in thepresence of steam and in another embodiment, the process is performed inthe absence of steam.

The present invention also relates to the use of the catalyticcomposition as described above for the dehydrogenation of light alkanes.

It had been found that the materials covered by the embodiment of theinvention disclosed in co-pending United States patent application Ser.No. 08/569,185 and described in more detail herein, predominantlymaintains the MgO structure, and also a high specific surface area andexhibited an improved stability towards sintering compared to thematerials reported in Schaper, H., et al., Appl. Cat., vol. 54, 79(1989) and Schaper, H., European Patent No. 0 251 351 (1988), supra.

Preferably, the catalyst has been subjected to a pretreatment comprisinga reduction, preferably in hydrogen, a subsequent oxidation, preferablyin air optionally mixed with nitrogen, and finally a second reduction,preferably in hydrogen (ROR pretreatment;ROR=Reduction-Oxidation-Reduction).

The Group VIII noble metal is preferably selected from platinum andpalladium, with platinum being the most preferred. The Group IVA metalis preferably selected from tin and germanium, with the most preferredmetal being tin.

It has further been shown that the selectivity of the catalysts of theinvention in a dehydrogenation process is further improved by includingtherein a Group IA alkali metal, preferably cesium or potassium, mostpreferably cesium.

It is remarkable that the new catalyst exhibits a very high activity inthe dehydrogenation of hydrocarbons even with a low content of GroupVIII noble metal of e.g. 0.2-0.4 wt. %.

The Group VIII metal, the Group IVA metal and the optional Group IAmetal can be incorporated into the carrier by any of the methods knownin the art. A preferred method consists in impregnating the oxidecarrier with solutions or suspensions of decomposable compounds of themetals to be incorporated.

The catalyst and its preparation are described in more detail below withreference to embodiments wherein platinum, tin and optionally cesium aredeposited on the carrier material, but the description is also valid forthe deposition of other metals within the scope of the invention, withany adaptations that will be obvious to a person skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The significance of the present invention can be better understood byreference to the drawings.

FIG. 1 shows steam stabilization tests at 650° C., materials prepared byMg/Al ratio 3 and 5 and calcination temperature 700 and 800° C.

FIG. 2 shows steam stabilization tests at 650° C., materials prepared byMg/Al ratio 3 and 5 and calcination temperature 800° C.

FIG. 3 shows steam stabilization tests at 650° C., materials prepared byMg/Al=3 and calcination temperature 700° C. by using NH₄ ⁺ or Na⁺ saltsin the precipitation of the materials.

FIG. 4 shows steam stabilization test and thermion stabilization test at650° C., materials prepared by Mg/Al=5 and calcination temperature 800°C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a dehydrogenation catalyst, a catalystsupport and a method of preparing and for using the catalyst. In oneembodiment of the invention, the catalyst includes a carrier, orcatalyst support, and a Group VIII noble metal, a Group IVA metal, andotionally a Group IA alkali metal. The carrier may be a mixed oxide ofmagnesium and aluminum, Mg(Al)O, which is characterized by a magnesiastructure wherein some of the magnesium atoms are replaced by aluminumatoms. The molar ratio of Mg to Al is typically ranging from 1:1 to10:1, and the surface area is typically ranging from 100 to 300 m²/g,preferably from 140 to 210 m²/g. The particle size can be in the rangeof 100 μm to 20 mm.

The deposition of platinum and tin on the Mg(Al)O carrier material canadvantageously be carried out in one step, e.g. by using tin chlorideand hexachloroplatinic acid dissolved in ethanol. A method fordepositing platinum and tin in a single step is described in J.Catalysis, vol. 128, 1 (1991). By carrying out a simultaneous depositionof platinum and tin on the Mg(Al)O material, the number of requiredcalcination steps is reduced, which makes it easier to obtain a highsurface area of the Mg(Al)O material. Other suitable impregnationprocedures are described in the above-mentioned U.S. Pat. No. 4,788,371,in U.S. Pat. No. 4,962,265 and in EP 0,098,622, each incorporated hereinby reference.

In cases where the catalyst contains cesium, a deposition of cesium canbe effected in a separate step, after the deposition of tin and platinumand the subsequent calcination, using cesium nitrate dissolved in water.The impregnation with cesium nitrate can be carried out as described inU.S. Pat. No. 4,788,371.

The Reduction Oxidation Reduction (ROR) pretreatment of the catalyst isconveniently effected by carrying out a reduction of the catalyst inhydrogen, a subsequent oxidation in air optionally mixed with nitrogen,and finally a second reduction in hydrogen. The pretreatment can becarried out at temperatures in the range of 500° to 700° C. and by usingspace velocities (GHSV) for the treatment gases of 10 to 100,000 N mlg⁻¹ h⁻¹, preferably 100 to 5000 N ml g⁻¹ h⁻¹. The initial reduction ofthe catalyst with hydrogen is carried out for a period of 1 minute to 10hours, usually for about 2 hours. The subsequent oxidation of thereduced catalyst in air optionally mixed with nitrogen is carried outfor a period of 1 minute to 10 hours, usually for about 2 hours. Theoxidation may advantageously be accomplished by first treating thecatalyst for about 1 hour in a stream of nitrogen containing about 20%by volume of air, and then treating it for about 1 hour in pure air. Thefinal reduction with hydrogen is carried out under similar conditions asthe initial reduction.

Thus, the invention also relates to a process for preparing theabove-described ROR pretreated catalyst. The process is characterized bythe steps of incorporating a Group VIII noble metal, a Group IVA metaland optionally a Group IA alkali metal into a carrier consistingessentially of a mixed oxide of magnesium and aluminum Mg(Al)O, andsubjecting the material thus obtained to ROR pretreatment comprising areduction, preferably in hydrogen, a subsequent oxidation, preferably inair optionally mixed with nitrogen, and finally a second reduction,preferably in hydrogen.

The invention further provides a process for dehydrogenatingdehydrogenatable C₂₋₃₀ hydrocarbons, preferably C₂₋₅ paraffins,comprising contacting the hydrocarbons, under suitable dehydrogenationconditions in one or more reaction zones, with a solid catalystcomprising a combination of a carrier, constituted essentially by amixed oxide of magnesium and aluminum Mg(Al)O, a Group VIII noble metal,a Group IVA metal and optionally a Group IA alkali metal.

In accordance with usual practice in the dehydrogenation ofhydrocarbons, the hydrocarbons are preferably contacted with the solidcatalyst in a gaseous phase, mixed with usual additives such as steam,nitrogen and hydrogen. The feed mixture containing the hydrocarbons ispreferably introduced into a reactor having one or more fixed catalystbeds, and the dehydrogenation is preferably carried out at a temperatureranging from 500° to 700° C., at a pressure ranging from 0.5 to 1.5 barsabsolute, and using a space velocity (GHSV) ranging from 10 to 10.000 Nml g⁻¹ h⁻¹.

The new catalyst has also been shown to be very suitable in cases wherethe dehydrogenation of hydrocarbons is carried out in combination withadmixing of oxygen and combustion of hydrogen, because the new catalystalso exhibits a selective catalytic effect on the oxidation of hydrogento water.

It is well known in the art of dehydrogenating dehydrogenatablehydrocarbons that it is advantageous to oxidize with anoxygen-containing gas the hydrogen formed in the reaction. Because thedehydrogenation process is endothermic, oxidation of the formed hydrogencan be utilized to maintain the desired reaction temperature during thedehydrogenation. For such heating purpose it will often be advantageouseven to add a supplementary amount of recirculated hydrogen to thereaction mixture. In addition to achieving a desired heat balance, thelowering of the hydrogen concentration in the reaction mixture resultingfrom the combustion will shift the equilibrium of the desireddehydrogenation reactions in the direction of higher yields ofunsaturated hydrocarbons. Although it will be advantageous for thatreason to achieve a high hydrogen conversion, it is important however toavoid excessive concurrent oxidation of hydrocarbons, which would reducethe total yield of the process. It is therefore important to achieve amaximum of selectivity of the oxidation of the hydrogen formed in thedehydrogenation process. It has been found that such selective oxidationis achieved with the new catalyst.

Thus, the invention also provides a process for dehydrogenatingdehydrogenatable C₂₋₃₀ hydrocarbons, preferably C₂₋₅ paraffins, incombination with admixing of an oxygen-containing gas, preferablyoxygen, and combustion of hydrogen, comprising contacting thehydrocarbons under suitable dehydrogenation conditions in one or morereaction zones, with a solid catalyst comprising a combination of acarrier, constituted essentially by a mixed oxide of magnesium andaluminum Mg(Al)O, a Group VIII noble metal, a Group IVA metal andoptionally a Group IA alkali metal.

In accordance with usual practice in such dehydrogenation ofhydrocarbons, the hydrocarbons are contacted with the solid catalyst ina gaseous phase, mixed with an oxygen-containing gas and with usualadditives such as steam, any supplementary quantities of hydrogen, andnitrogen. The feed mixture containing the hydrocarbons is preferablyintroduced into a reactor having one or more fixed catalyst beds, withoxygen-containing gas being introduced and admixed with the feed streameven between the catalyst beds when more than one such bed is used. Thedehydrogenation is preferably carried out at a temperature ranging from400° to 700° C., at a pressure ranging from 0.5 to 3 bars absolute, andusing a space velocity (GHSV) ranging from 10 to 10.000 N ml g⁻¹ h⁻¹.

In both of the two types of the dehydrogenation process the activity ofthe catalyst will decrease with time. When the activity has becomeundesirably low, the catalyst may be regenerated, e.g. in the samereactor. The regeneration can be carried out by burning off the cokethat has been formed on the catalyst, with an oxygen-containing gas fora period of time ranging from 1 minute to 10 hours, preferably in astream of air optionally mixed with nitrogen. The catalyst is thensubjected to a reduction treatment for a period of 1 minute to 10 hoursin a stream of hydrogen. Said treatments are suitably carried out at300° to 700° C. using a space velocity (GHSV) for the treatment streamsof 10 to 10,000 N ml g⁻¹ h⁻¹, preferably 100 to 5000 N ml g⁻¹ h⁻¹. Ifdesired, a redispersion of the noble metal, e.g. platinum, in thecatalyst can be effected using a chlorine-containing gas after theburning off of the coke but prior to the reduction treatment.

The regeneration of the catalyst restores to a substantial extent theoriginal characteristics of the catalyst. The restoration of theactivity and the selectivity of the catalyst will be more complete inthe temperature range of 300° C. to 400° C. than at the highertemperatures. Admixing nitrogen with the air stream utilized for theoxidation also tends to improve the restoration of the properties of thecatalyst.

Compared to the previously known dehydrogenation catalysts on an aluminabase, the new catalyst exhibits improved activity and improvedstability.

EXAMPLE 1

A Mg(Al)O material having an atomic ratio of Mg to Al of 2:1 to 3:1 wasprepared according to the following procedure: An aqueous solution of1.13 moles of NaOH and 0.045 mole of Na₂CO₃ was treated with a solutionof 0.91 mole of Mg(NO₃)₂6H₂O and 0.09 mole of Al(NO₃)₃9H₂O at about 75°C. (pH=9.5). After filtration, washing and drying at about 100° C. forabout 15 hours, a hydrotalcite Mg₆Al₂(OH)₁₆CO₃4H₂O was formed. Thestructure was confirmed by X-ray diffraction analysis. The material thusobtained was calcined at 700° C. for about 15 hours, whereby Mg(Al)O wasformed. The structure was confirmed by X-ray diffraction analysis, andthe surface area was measured to be 156 m²/g.

EXAMPLE 2

A Mg(Al)O material having an atomic ratio of Mg to Al of 2:1 to 3:1 wasprepared according to the following procedure: An aqueous solution of1.13 moles of NH₄OH and 0.045 mole of (NH₄)₂CO₃ was treated with asolution of 0.91 mole of Mg(NO₃)₂6H₂O and 0.09 mole of Al(NO₃)₃9H₂O at atemperature of about 75° C. (pH=9.5). After filtration, washing anddrying at about 100° C. for about 15 hours, a hydrotalciteMg₆Al₂(OH)₁₆CO₃4H₂O was formed. The material thus obtained was calcinedat 700° C. for about 15 hours, whereby Mg(Al)O was formed. The structurewas confirmed by X-ray diffraction analysis, and the surface area wasmeasured to be 198 m²/g.

EXAMPLE 3

A Mg(Al)O material having a particle size of 300-400 μm, preparedaccording to Example 1, was impregnated with a solution containing tinchloride and hexachloroplatinic acid and with a solution of cesiumnitrate, according to the following procedure:

0.1150 g SnCl₂2H₂O and 0.0805 g H₂PtCl₆6H₂O were dissolved in 60 ml ofethanol and the mixture was added to 10.1 g of Mg(Al)O. After completionof the impregnation the material thus obtained was evaporated to drynessin a vacuum and was then dried at about 100° C. for about 15 hours,whereupon the dried material was calcined at 560° C. for about 3 hoursin air supplied in an amount of 100 cm³/min.

0.0711 g CsNO₃ dissolved in 25 ml of water was then added to thecalcined material. Upon completion of the impregnation, the materialthus obtained was dried at about 100° C. for about 15 hours. The driedmaterial was calcined at 560° C. for about 3 hours in air supplied in anamount of 100 cm³/min.

3 g of the calcined product were then reduced at 600° C. for 2 hours ina stream of H₂ supplied in an amount of 20 cm³/min.

The reduced product was then oxidized at 600° C. for about 1 hour in astream of N₂ containing 20% by volume of air, added in an amount of 50cm³/min, and for about 1 hour in pure air supplied in an amount of 50cm³/min. The oxidized product was then reduced in the same manner asbefore the oxidation.

A catalyst was obtained which had the following chemical composition:

0.3 wt. % Pt

0.6 wt. % Sn

0.5 wt. % Cs

98.6 wt. % Mg(Al)O.

The catalyst was tested for a dehydrogenation of propane in amicroreactor equipped with a fixed catalyst bed, at the followingconditions:

Dehydrogenation temperature: 600° C. Dehydrogenation pressure: 1 barabs. Space velocity (GHSV): 2100 N ml g⁻¹ h⁻¹ Amount of catalyst: 3.0 gComposition of the feed stream: Propane 35 Nml/min Hydrogen  5 Nml/minNitrogen 25 Nml/min Steam 41 Nml/min The results are given in Table 1.

EXAMPLE 4 Comparison Example

The procedure of Example 3 was repeated, with the following exception:After the first reduction of the calcined product with H₂, the oxidationin air-containing N₂ and the subsequent second reduction with H₂ wereomitted.

The catalyst was used for a dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

EXAMPLE 5

The procedure of Example 3 was followed, including the post-treatmentconsisting in a reduction, a subsequent oxidation, and a secondreduction (ROR pretreatment) of the calcined catalyst, but theimpregnation with CsNO₃ for incorporation of cesium was omitted. Theimpregnation with a solution containing tin chloride andhexachloroplatinic acid was accomplished in the presence of quantitiesof tin chloride and hexachloroplatinic acid resulting in a catalysthaving the chemical composition:

0.3 wt. % Pt

0.6 wt. % Sn

99.1 wt. % Mg(Al)O.

The catalyst was used for a dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

EXAMPLE 6

A Mg(Al)O material having a particle size of 300-400 μm, preparedaccording to Example 2, was impregnated with a solution containing tinchloride and hexachloroplatinic acid according to the followingprocedure:

0.1150 g SnCl₂2H₂O and 0.0805 g H₂PtCl₆6H₂O were dissolved in 60 ml ofethanol and the mixture was added to 10.1 g of Mg(Al)O. After completionof the impregnation the material thus obtained was evaporated to drynessin a vacuum and then dried at about 100° C. for about 15 hours,whereupon the dried material was calcined at 560° C. for about 3 hoursin air supplied in an amount of 100 cm³/min.

3 g of the calcined product were then reduced at 600° C. for 2 hours ina stream of H₂ supplied in an amount of 20 cm³/min.

The reduced product was thereafter oxidized at 600° C. for about 1 hourin a stream of N₂ containing 20% by volume of air, supplied in an amountof 50 cm³/min, and for about 1 hour in pure air supplied in an amount of50 cm³/min. The oxidized product was then reduced in the same manner asbefore the oxidation.

A catalyst was obtained which had the following chemical composition:

0.3 wt. % Pt

0.6 wt. % Sn

99.1 wt. % Mg(Al)O.

The catalyst was used for a dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

EXAMPLE 7

The procedure of Example 6 was followed, including the post-treatmentconsisting in a reduction, a subsequent oxidation, and a secondreduction (ROR pretreatment) of the calcined catalyst, but theimpregnation with a solution containing tin chloride andhexachloroplatinic acid was accomplished in the presence of quantitiesof tin chloride and hexachloroplatinum acid resulting in a catalysthaving the chemical composition:

0.3 wt. % Pt

0.9 wt. % Sn

98.8 wt. % Mg(Al)O.

The catalyst was used in dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

EXAMPLE 8

The procedure of Example 6 was followed, including the post-treatmentconsisting in a reduction, a subsequent oxidation, and a secondreduction (ROR pretreatment) of the calcined catalyst, but theimpregnation with a solution containing tin chloride andhexachloroplatinic acid was accomplished in the presence of suchquantities of tin chloride and hexachloroplatinum acid that a catalystwas obtained having the chemical composition:

0.3 wt. % Pt

1.2 wt. % Sn

98.5 wt. % Mg(Al)O.

The catalyst was used for a dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

EXAMPLE 9 Comparison Example

A known dehydrogenation catalyst was prepared according to the processdisclosed in U.S. Pat. No. 4,788,371. 0.179 g of SnCl₂2H₂O dissolved in14 ml of water was added to 18.8 g of θ-alumina having a particle sizeof 100 to 400 μm. After completion of the impregnation, the resultingmaterial was dried at about 100° C. for about 6 hours. The driedmaterial was calcined for about 3 hours at 600° C. in a stream of airsupplied in an amount of 100 cm³/min.

0.349 g of H₂PtCl₆6H₂O dissolved in 14 ml of water was added to thecalcined material. After completion of the impregnation, the resultingmaterial was dried at about 100° C. for about 15 hours. The driedmaterial was calcined for a period of 3 hours at 570° C. in a stream ofair containing 10% of steam and supplied in an amount of about 100cm³/min.

1.06 g of CsNO₃ dissolved in 14 ml of water were added to the calcinedmaterial. Upon completion of the impregnation, the resulting materialwas dried at about 100° C. for about 30 hours. The dried material wascalcined for about 3 hours at 570° C. in an air stream supplied in anamount of about 100 cm³/min.

3 g of the obtained catalyst were then reduced at 600° C. for about 2hours in a stream of H₂ supplied in an amount of 20 cm³/min.

A catalyst was obtained having the chemical composition:

0.7 wt. % Pt

0.5 wt. % Sn

3.9 wt. % Cs

94.9 wt. % θ-alumina.

The catalyst was used for a dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

EXAMPLE 10 Comparison Example

A catalyst was prepared according to Example 9, whereupon 3 g of thereduced catalyst were oxidized at 600° C. for about 1 hour in a streamof N₂ containing 20% by volume of air, supplied in an amount of 50cm³/min, and for about 1 hour in pure air supplied in an amount of 50cm³/min. The oxidized product was then reduced in the same manner asbefore the oxidation, i.e. at 600° C. for a period of 2 hours in astream of H₂ supplied in an amount of 20 cm³/min.

Thus, the post-treatment of the catalyst accomplished after thecalcination corresponded to a ROR pretreatment as prescribed for thecatalysts of the invention.

The catalyst was used for a dehydrogenation of propane under the sameconditions as in Example 3. The results obtained are given in Table 1.

TABLE 1 Dehydrogenation of propane to propene. Metal Conv. of Conv. ofC-sel. to C-sel. to Yield content propane propane propene propenepropene⁵ Example Carrier (wt. %) 5 h (%) 25 h (%) 5 h (%) 25 h (%) 25 h(%) 3 Mg(Al)O 0.3 Pt 52.8 45.6 97.5 97.8 44.6 0.6 Sn 0.5 Cs 4¹ Mg(Al)O0.3 Pt 19.3 16.7 97.8 97.3 16.2 Comp. Cat. 0.6 Sn 0.5 Cs 5 Mg(Al)O 0.3Pt 58.7 53.0 93.3 97.3 51.6 0.6 Sn 6² Mg(Al)O 0.3 Pt 58.8 57.5 93.0 95.955.1 0.6 Sn 7² Mg(Al)O 0.3 Pt 58.0 57.8 93.9 96.1 55.5 0.9 Sn 8² Mg(Al)O0.3 Pt 58.6 57.5 94.9 95.9 55.1 1.2 Sn 9³ θ-Al₂O₃ 0.7 Pt 38.0 27.0 97.095.0 25.7 Comp. Cat. 0.5 Sn 3.9 Cs 10⁴ θ-Al₂O₃ 0.7 Pt 41.4 31.0 96.495.9 29.7 Comp. Cat. 0.5 Sn 3.9 Cs ¹Without ROR pretreatment. ²Mg(Al)Ohaving a large surface area (198 m²/g). ³A catalyst known from U.S. Pat.No. 4,788,371. ⁴A catalyst known from U.S. Pat. No. 4,788,371 butsubjected to a ROR pretreatment.${\quad^{5}{Yield}\quad {of}\quad {propene}} = \frac{{Number}\quad {of}\quad {moles}\quad {of}\quad C\quad {as}\quad C_{3}H_{6}}{\begin{matrix}{{{Number}\quad {of}\quad {moles}\quad {of}\quad C\quad {as}\quad C_{3}H_{8}} +} \\{{Number}\quad {of}\quad {moles}\quad {of}\quad C\quad {in}\quad {products}}\end{matrix}}$

The results in Table 1 show that a ROR pretreated catalyst of theinvention provides a large increase in the propane conversion comparedto a similar catalyst not having been subjected to such pretreatment(Example 3 compared to Example 4). The selectivity for forming propeneis retained at about the same level, whereby the total yield of propeneis substantially increased.

The results in Table 1 also show that an increase in the surface area ofthe Mg(Al)O material from 156 m²/g to 198 m²/g results in a somewhatmore stable catalyst and consequently in an increased yield of propeneafter 25 hours (Example 6 compared to Example 5).

An increase of the catalysts' content of Sn from 0.6 wt. % to 0.9 wt. %appears to result in a further increased yield of propene (Example 7compared to Example 6).

The previously known catalyst of Example 9 gives a substantially loweryield of propene than the new catalysts (Examples 3, 5, 6, 7, 8). Whenthe previously known catalyst of Example 9 is subjected to a completeROR pretreatment as prescribed according to the invention (Example 10),the yield is improved even for said previously known catalyst.Nonetheless, the improving effect of the ROR pretreatment is not nearlyas good for the known catalyst as for the new catalysts. Thus, the newcatalysts also give a substantially better yield of propane than the RORpretreated catalyst of Example 10.

EXAMPLE 11

The performance of one of the new catalysts of the invention wascompared to the performance of a previously known catalyst for adehydrogenation of propane accomplished in combination with combustionof hydrogen with an oxygen-containing gas. The combination ofdehydrogenation and hydrogen combustion was carried out in a reactorcomprising two catalyst zones and an intermediary oxygen admixing zone.In addition to oxygen being added to the feed to the first catalystzone, oxygen was also introduced into said oxygen admixing zone betweensaid two catalyst zones.

The new catalyst (I) consisted of 0.3 wt. % Pt and 1.2 wt. % Sn onMg(Al)O and was a catalyst similar to the one of Example 8 above, exceptthat it had been prepared with a particle size of 1-2 mm.

The known catalyst (II) was a catalyst according to U.S. Pat. No.4,788,371, consisting of 0.65 wt. % Pt. 1.15 wt. % Sn and 2.18 wt. % Cson θ-alumina. Catalyst (II) had been prepared according to said U.S.Pat. No. 4,788,371, as described in Example 9 above, except thatsimilarly with the new catalyst (I) it had been prepared with a particlesize of 1-2 mm.

The conditions employed in the combined dehydrogenation and hydrogencombustion, and the results obtained, are summarized in the followingTable 2.

The results in Table 2 show that the new catalyst I and the knowncatalyst II, which are both described in Example 11, are both capable ofachieving a selective oxidation of the hydrogen in the gas mixture.

The conversion of propane C₃H₈, and thus the yield of propene C₃H₆, issubstantially higher for the new catalyst than for the known catalyst,viz. 57% versus 45% after 5 hours, and 55% versus 30% after 20 hours ofoperation, respectively. The higher propane yield was achieved in spiteof the fact that the gas space velocity per gram of catalyst and perhour (GHSV) was higher for the new catalyst (2100 N ml g⁻¹ h⁻¹ versus1400 N ml g⁻¹ h⁻¹ for the known catalyst), and in spite of the fact thatthe content of active noble metal (platinum) was substantially lower inthe new catalyst (0.3 wt. % versus 0.65 wt. % in the known catalyst).The higher (GHSV) used with the new catalyst was due to the fact thatthis catalyst had a lower bulk weight than the known catalyst. As aconsequence of the lower bulk weight of the new catalyst, the advantageresulting from its low content of platinum was even more important thansuggested by the perceptual content alone. A low content of platinum ina commercial catalyst is important from an economical point of view.

The selectivity for oxidation of hydrogen to water is somewhat higherfor the known catalyst than for the new catalyst, viz. 88% versus 80%after 5 hours, and 95% versus 87% after 20 hours of operation,respectively. This may be explained at least partly by the fact that thelower propane conversion achieved by the known catalyst resulted in theformation of lesser amounts of the desired dehydrogenated product,propane. Thus, with the known catalyst the oxidation of hydrogen towater was less burdened by competing oxidation of propene to carbonoxides.

Improvements in Stability

It has been found that improved stability is obtained using catalystsupport materials comprised of a mixed oxide which consists essentiallyof a divalent metal and a trivalent metal in a substantially homogeneousphase. The mixed oxide is a calcination product of a hydrotalcite-likephase calcinated at a temperature of about 700-1200° C., wherein thedivalent metal/trivalent metal molar ratio is greater than or equal to2. The divalent/trivalent metals are preferably Mg and Al, with theMg/Al molar ratio of the catalyst support material preferably being inthe range of about 3 to about 5.

The catalyst is a dehydrogenation catalyst comprised of one or both of atransition metal selected from the first row of transition metals of thePeriodic Table, or a Group VIII metal impregnated onto the catalystsupport material. The first row transition metal is preferably Cr. Thecatalyst preferably has a Group IVA metal and optionally a Group 1Ametal impregnated together with a Group VIII metal onto the catalystsupport material described above. The Group VIII metal is preferably Pt.The Group IVA metal is preferably Sn, and the Group IA metal ispreferably Cs. The amount of the Group VIII metal is preferably0.05-5.0% by weight, the amount of the Group IVA metal is preferably0.05-7.0% by weight, and the amount of the optional Group IV metal ispreferably 0.05-5.0% by weight.

To prepare the catalyst support material, a solution comprising adivalent metal salt and a trivalent metal salt is mixed with a basicaqueous solution, preferably a composition of aqueous ammoniumhydroxides and carbonates. The reaction product is recovered from thereaction mixture and product is washed and dried. The dried product iscalcinated at a temperature ranging from 700-1200° C., preferably about750-950° C., more preferably at about 770-850° C. and most preferably atabout 800° C.

The steam testing discussed later in the following examples shows thatmaterials calcinated at 800° C. after 72 hours have a higher specificsurface area than those calcinated at 700° C. (See FIG. 1.) Thisinvolves a less frequent change of catalyst in the dehydrogenationprocess of light alkanes to alkenes. This less frequent changing ofcatalyst is of great importance when running a dehydrogenation reactionin industrial plants. This surprising improvement was not to be expectedfrom the earlier catalyst calcined at 700° C. described above and insome examples set forth herein.

The following Examples are set forth to illustrate the inventiondisclosed herein. These examples should not, however, be construed aslimiting the scope of the novel invention:

General

Calcination was performed under flowing air (100 ml/min). The sample(5-50 g) was heated with a heating rate of 3° C./min to the finalcalcination temperature. After completing the calcination, the samplewas cooled with a cooling rate of approximately 2° C./min.

Specific surface area was measured using nitrogen by the BET method. Themeasurement accuracy was ±5%. Powder XRD (Siemens D-5000 diffractometerwith Cu—K_(α)radiation) was used to check crystallinity.

EXAMPLE 12

A Mg(Al)O material having an atomic ratio of Mg to Al of 3:1 wasprepared according to the following procedure: An aqueous solution of0.55 mole of NH₄OH and 0.045 mole of (NH₄)₂CO₃ was treated with asolution of 0.91 mole of Mg(NO₃)₂6H₂O and 0.09 mole of Al(NO₃)₃9H₂O at atemperature of about 60° C. (pH=9). After filtration, washing and dryingat about 100° C. for about 15 hours, a hydrotalcite Mg₆Al₂(OH)₁₆CO₃4H₂Owas formed. The material thus obtained was calcined at 700° C. for about15 hours, whereby Mg(Al)O was formed. The structure was confirmed byX-ray diffraction analysis, and the surface area was measured to be 176m²/g.

EXAMPLE 13

A Mg(Al)O material having an atomic ratio of Mg to Al of 3:1 wasprepared according to the following procedure: An aqueous solution of1.13 moles of NaOH and 0.045 mole of Na₂CO₃ was treated with a solutionof 0.91 mole of Mg(NO₃)₂6H₂O and 0.09 mole of Al(NO₃)₃9H₂O. Afterfiltration, washing and drying at about 100° C. for about 15 hours, ahydrotalcite Mg₆Al₂(OH)₁₆CO₃4H₂O was formed. The structure was confirmedby X-ray diffraction analysis.

a) The material thus obtained was calcined at about 700° C. for about 15hours, whereby Mg(Al)O was formed. The structure was confirmed by X-raydiffraction analysis, and the surface area was measured to be 187 m²/g.

b) The material thus obtained was calcined at about 800° C. for about 15hours, whereby Mg(Al)O was formed. The structure was confirmed by X-raydiffraction analysis, and the surface area was measured to be 162 m²/g.

c) The material thus obtained was calcined at about 900° C. for about 15hours, whereby Mg(Al)O was formed, together with traces of inverseMgAl₂O₄ spinel. The structure was confirmed by X-ray diffractionanalysis, and the surface area was measured to be 110 m²/g.

d) The material thus obtained was calcined at about 1000° C. for about15 hours, whereby Mg(Al)O was formed, together with some inverse MgAl₂O₄spinel. The structure was confirmed by X-ray diffraction analysis, andthe surface area was measured to be 61 m²/g.

EXAMPLE 14

A Mg(Al)O material having an atomic ratio of Mg to Al of 5:1 wasprepared according to the following procedure: An aqueous solution of1.13 moles of NaOH and 0.045 mole of Na₂CO₃ was treated with a solutionof 0.91 mole of Mg(NO₃)₂6H₂O and 0.09 mole of Al(NO₃)₃9H₂O. Afterfiltration, washing and drying at about 100° C. for about 15 hours, ahydrotalcite Mg₆Al₂(OH)₁₆CO₃4H₂O was formed. The structure was confirmedby X-ray diffraction analysis.

a) The material thus obtained was calcined at about 700° C. for about 15hours, whereby Mg(Al)O was formed. The structure was confirmed by X-raydiffraction analysis, and the surface area was measured to be 169 m²/g.

b) The material thus obtained was calcined at about 800° C. for about 15hours, whereby Mg(Al)O was formed. The structure was confirmed by X-raydiffraction analysis, and the surface area was measured to be 157 m²/g.

EXAMPLE 15

To investigate their steam stability, the support materials obtained asdescribed in Examples 12, 13a) and 14a) were tested in a fluidized bedquartz apparatus. The steam stability test procedure was as follows:

The material was loaded into the reactor, which was then heated to 600°C. under a N₂ flow. When 600° C. was reached, steam was added to thefeed. Such conditions (600° C., 50%H₂O/50%/N₂) were maintained for 22hours. A sample of the material was then withdrawn from the reactor, andthe temperature increased to 650° C. Such conditions (650° C.,50%H₂O/50%N₂) were maintained for 48 hours. New samples were withdrawnat 650° C. After completion of the test, the steam feed was turned off,and the reactor cooled to 25° C. under a N₂ flow. The remainder of thematerial was then collected. The sample materials were analyzed by BETand XRD.

The steam stability test results are shown in FIGS. 1 and 3. (Example 12is only included in FIG. 3). The surface area in m²/g plotted along thevertical axis is expressed as a function of the duration of the steamstability test in hours (plotted along the horizontal axis).

EXAMPLE 16

To investigate the influence of calcination temperature on thematerials' steam stability, the support materials obtained as describedin Examples 13b) and 14b) were tested in a fluidized bed quartzapparatus, according to Example 15, but with a prolonged test durationat 650° C. (314 hours).

The results for the whole test (336 hours) are shown in FIG. 2. The testfor the first 72 hours is shown in FIG. 1 together with the results fromExample 15. The surface area in m²/g plotted along the vertical axis isexpressed as a function of the duration of the steam stability test inhours (plotted along the horizontal axis).

EXAMPLE 17

To investigate its thermal stability, the support material obtained asdescribed in Example 14b was tested as described in Example 16, exceptthat 100% N₂ was used as feed gas during the whole test. The thermalstability test results are shown in FIG. 4.

EXAMPLE 18

A Mg(Al)O material having a particle size less than 100 μm, preparedaccording to any one of Example 12-14, was impregnated with a solutioncontaining tin chloride and hexachloroplatinic acid according to thefollowing procedure:

0.2304 g SnCl₂2H₂O and 0.0805 g H₂PtCl₆6H₂O were dissolved in 80 ml ofethanol and the mixture was added to 10.1 g of Mg(Al)O. After completionof the impregnation the material thus obtained was evaporated to drynessin a vacuum and was then dried at about 100° C. for about 15 hours,whereupon the dried material was calcined at 560° C. for about 3 hoursin air supplied in an amount of 100 cm³/min.

EXAMPLE 19

To investigate its steam stability, a material prepared as described inExample 18, with Mg/Al ratio of 3, and which had been calcined at 700°C. prior to impregnation, was tested as described in Example 15. Thesteam stability test results are shown in FIG. 1.

EXAMPLE 20

To investigate its stability during catalytic testing, two materialsprepared as described in Example 18, with a Mg/Al ratio of 3, and whichhad been calcined at 700 or 800° C. prior to impregnation, werepelletized by pressing, crushing and sieving to a pellet size of 0.7-1.0mm, and tested as a catalyst for propane dehydrogenation. The tests wereperformed in a titanium laboratory scale fixed bed reactor with an innerdiameter of 9 mm. A titanium tube with an outer diameter of 3 mm waslocated in the center of the reactor. The catalyst pellets were placedon a titanium sinter in the reactor. The reactor temperature wascontrolled by a thermocouple placed in the tube inside the reactor. Thetotal pressure in the reactor was 1.1 bar. The catalysts (appx. 1 g)were tested under the following conditions: T=600-620° C., GHSV−600 h⁻¹and C₃H₆:H₂O=1:2 (mole basis). The BET surface area measured before andafter testing is shown in Table 3.

TABLE 3 Catalytic Testing Calcination BET surface area (m²/g),temperature Test duration after: (° C.) (h) Pelletization Testing 700380 124 112 800 380 131 121

The results presented in the Examples show that the initial surface areaof the calcined materials decreases with increasing calcinationtemperature. The results presented in FIGS. 1 and 2 further show thatthe materials calcined at a higher temperature maintain a higherspecific surface area during subsequent steam testing at 600-650° C.Indeed, for a steam test duration of more than 72 hours, the materialscalcined at 800° C. have a higher specific surface area than thosecalcined at 700° C.

It is further observed that the initial specific surface area of thematerials calcined at a certain temperature, decreases with anincreasing Mg/Al ratio. During subsequent steam testing for 336 hoursthe material with a higher Mg/Al ratio maintain a higher specificsurface area compared to the material with a lower Mg/Al ratio (FIG. 2).Indeed, after 50 hours of steam testing, the order of specific surfacearea is reversed, so that the materials with a higher Mg/Al ratio have ahigher specific surface area than those with a lower Mg/Al ratio (FIG.1).

Preparation of the material with NH₄ ⁺ instead of Na⁺ precursor led to amaterial with a slightly lower initial specific surface area but with ahigher stability during steam testing (FIG. 3).

Impregnation of a calcined material with Pt and Sn led to a decrease inthe initial specific surface area of that material. The subsequentdecrease in specific surface area during steam testing was similar tothat observed for the fresh material (FIG. 1).

Pelletization of the impregnated materials led to a decrease in theirspecific surface area (Table 3). Subsequent testing of the material as apropane dehydrogenation catalyst showed that the excellent surface areastability observed for these materials during steam testing, is alsovalid under catalytic test conditions. Even here, an improved stabilitywas indicated for the material calcined at 800° C.

Finally, it is observed that the presence of steam is an importantfactor for the thermal stability of the materials covered by thisinvention: When no steam was added to the feed the specific surface areaof a Mg/Al=5 material calcined at 800° C. was stable throughout a 334hours test at 600-650° C. (FIG. 4).

This means that to prevent sintering of the catalyst support during thedehydrogenation of alkanes, the dehydrogenation can advantageously beperformed without steam.

What is claimed is:
 1. A process for the catalytic dehydrogenation of alight alkane, wherein a stream of a light alkane is passed through alayer of the catalyst, the catalyst comprising platinum and tinimpregnated onto a catalyst support material comprising a mixed oxideconsisting essentially of an oxide of a divalent metal and a trivalentmetal in a substantially homogeneous phase, which mixed oxide is acalcination product of a hydrotalcite-like phase calcinated at atemperature of about 770° C. to about 1200° C., wherein the divalentmetal to trivalent metal molar ratio is greater than or equal to 2, andwherein the platinum and tin contents of said hydrogenation catalystrange from 0.05% to 5% by weight and 0.05% to 7% by weight,respectively.
 2. The process of claim 1 wherein said catalyticdehydrogenation is performed in the presence of steam.
 3. The process ofclaim 1 wherein said catalytic dehydrogenation is performed in theabsence of steam.
 4. The process of claim 1 wherein said light alkane isa C₂₋₅ alkane.
 5. The process of claim 1 wherein said catalyticdehydrogenation is carried out at a temperature ranging from 400° C. to700° C.
 6. The process of claim 1 wherein the divalent metal is Mg andthe trivalent metal is Al.
 7. The process of claim 6 wherein the molarratio of Mg to Al is in the range of about 2.5 to about 6.0.
 8. Theprocess of claim 7 wherein the molar ratio of Mg to Al is in the rangeof about 3 to about
 5. 9. The process of claim 1 wherein thehydrotalcite-like phase is calcinated at a temperature of about 770° C.to about 850° C.
 10. The process of claim 1 wherein thehydrotalcite-like phase has been calcinated at a temperature of about800° C.
 11. The process of claim 1 wherein the hydrotalcite-like phasehas been calcinated at a temperature of about 770° C. to about 1200° C.for about 1 to about 20 hours.
 12. The process of claim 1 wherein thecatalyst has been subjected to a first reduction, an oxidation, and asecond reduction.